Definition of binary form

: a two-part musical form in which the first part modulates to the dominant or relative minor and the second returns to the tonic

In binary form, the opening melody returns in a key other than the tonic or does not return at all.

—John A. Rice, Antonio Salieri and Viennese Opera, 1998

In music of the Renaissance and Baroque periods, dances were often written in binary form; and in the collections of idealized dances … that became popular in the Baroque, binary form was increasingly used.